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The Genesis gold foil is a bulk solar wind collector, integrating fluences from all three of the wind regimes. Pyrolytic extraction of small foil samples at Minnesota yielded He fluences, corrected for backscatter, in good agreement with measurements by on-board spacecraft instruments, and He/Ne elemental ratios close to those implanted in collector foils deployed on the lunar surface during the Apollo missions. Isotopic distributions of He, Ne and Ar are under study. Pyrolysis to temperatures above the gold melting point generates nitrogen blanks large enough to obscure the solar-wind nitrogen component. An alternative technique for nitrogen and noble gas extraction, by room-temperature amalgamation of the gold foil surface, will be discussed. Ne and Ar releases in preliminary tests of this technique on small foil samples were close to 100% of the amounts expected from the high-temperature pyrolysis yields, indicating that amalgamation quantitatively liberates gases from several hundred angstroms deep in the gold, beyond the implantation depth of most of the solar wind. Present work is focused on two problems currently interfering with accurate nitrogen measurements at the required picogram to sub-picogram levels: a higher than expected blank likely due to tiny air bubbles rolled into the gold sheet during fabrication, and the presence of a refractory hydrocarbon film on Genesis collector surfaces (the "brown stain") that, if left in place on the foil, shields the underlying gold from mercury attack. We have found, however, that the film is efficiently removed within tens of seconds by oxygen plasma ashing. Potential nitrogen contaminants introduced during the crash of the sample return canister are inert in amalgamation, and so are not hazards to the measurements.
Reference:
Schlutter, D. J., Pepin, R. O., Extraction of Solar Wind Nitrogen and Noble Gases From the Genesis Gold Foil Collector
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Gold foil collector being cleaned at JSC during assembly of canister pre-launch |
Gold foil pre-launch |
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The Genesis Concentrator is designed to concentrate the heavy
ion flux from the solar wind by an average factor of at
least 20 and implant it into a target of ultra-pure, well-characterized
materials. High-transparency grids held at high voltages
are used near the aperture to reject >90% of the protons,
avoiding damage to the target. Another set of grids and
applied voltages are used to accelerate and focus the
remaining ions to implant into the target. The design
uses an energy-independent parabolic ion mirror to focus
ions onto a 6.2 cm diameter target of materials selected
to contain levels of O and other elements of interest
established and documented to be below 10% of the levels
expected from the solar wind. To optimize the concentration
of the ions, voltages are constantly adjusted based on
real-time solar wind speed and temperature measurements
from the Genesis ion monitor. Construction of the Concentrator
required new developments in ion optics; materials; and
instrument testing and handling.
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Concentrator target post return |
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Section of concentrator target post return |
Recovered concentrator target post return |
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Hexagonal collector
array used to capture
bulk solar wind |
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To successfully capture raw solar wind, Genesis flew a
million miles away, outside Earth's magnetic field, which
alters the particles, and hovered in its own orbit for
29 months. Scientists grappled with several challenges
while pondering ways to keep the samples pristine during
and after collection. First off, they had to find a proper
way to collect and transport the samples. The largest
collector consists of five bicycle-tire-sized collector
arrays, each loaded with 54 or 55 hexagonal wafers measuring
about 4 inches (10 centimeters) in diameter. These wafers
consist of 15 different high-purity materials including
aluminum, sapphire, silicon, germanium, gold and diamond-like
amorphous carbon — all chosen for their durability,
purity, cleanliness, retentiveness and ease of analysis.
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JSC Curation Team member displays some of the silicon wafer fragments collected |
Each collector array was assigned to catch various
types of solar wind. Genesis's goal was to collect billions
of atoms of solar particles heavier than hyrogen, equivalent
to "a few grains of salt," according to Genesis Principal
Investigator Dr. Donald Burnett of the California Institute
of Technology.
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Assortment of recovered fragments recovered in Utah |
Once the solar wind particles were collected, the wafers
had to be able to retain them while warming under the
Sun's rays. Each type of wafer will retain different
solar wind elements. Sapphire was used because it can
retain sodium under these conditions. Silicon, which
comprises approximately half of the materials used in
the collector arrays, does not retain sodium but does
retain many other elements, including the important
rock-forming element magnesium.
Geometry was also used to enhance researchers' ability
to analyze the sample. By making some of the collector
materials thin and mounting them on a rigid, inert structure
(e.g. silicon on sapphire), the effects of impurities
in the collector material were minimized by only analyzing
the thin layer. |
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There
are two solar wind spectrometers on-board the Genesis
spacecraft: the Genesis Ion Monitor (GIM) and the Genesis
Electron Monitor (GEM). The primary purpose of these
spectrometers is to enable the collection of appropriate
samples of the solar wind by the Genesis sample collectors.
This involves determining the type of solar wind that
is flowing past the spacecraft, adjusting high-voltages
in the Concentrator for the current conditions and deploying
the appropriate Collector Array for the type of solar
wind present, all in realtime. The secondary function
of the Monitors is to obtain high-quality solar wind
data that can be used for various scientific studies.
Interested parties are referred to the detailed instrument
description paper by Barraclough et al. (2003), listed
on the publications page.
The Genesis Ion Monitor (GIM)
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Genesis Ion
Monitor
(GIM) |
GIM consists of a 120° spherical-section electrostatic
analyzer (ESA) followed by an array of eight channel
electron multipliers (CEMs) for energy and angle analysis
of incoming ions. The ESA is negatively biased by a
high-voltage power supply that steps across a number
of voltage levels to build up an energy spectrum of
the plasma population. The GIM is basically an E/q analyzer
but does have a mass analysis capability in this instance
due to the similar flow velocities of all ions in the
solar wind beam. The energy range of the instrument
is ~100 eV to 14 keV, with a resolution of 5.2%, but
only a small fraction of this range is used at any one
time. Onboard software tracks the solar wind flow speed
and autonomously adjusts the energy range that is scanned
to keep it centered on the beam.
GIM has a field-of-view (FOV) that is ~3.0° in azimuth
by ~26° in polar angle and one of the narrow edges
of the FOV is aligned such that it slightly overlaps
the spacecraft rotational axis. Given this configuration,
during one spin of the spacecraft GIM sweeps out a circle
on the sky that is ~24° in radius with the center
of the circle being coincident with the average solar
wind flow direction at 1 AU. During each spin, GIM steps
forty times across ten individual energy steps and this
process is repeated for four spins of the spacecraft
with the energy steps being varied for each spin. These
four spins comprise a complete data cycle and require
approximately 2.5 minutes to complete. Thus GIM acquires
a complete measurement of the ion distribution function
every 2.5 minutes. The data product consists of ion
counts for eight polar angle and forty azimuthal angles
and forty energy levels.
The Genesis Electron Monitor (GEM)
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| Genesis Electron Monitor (GEM) |
The GEM sensor head is almost an identical copy of the electron spectrometers that are currently flying on ACE
(SWEPAM-E) and on Ulysses (BAM-E) but the electronics
are of a new design. Basically, GEM consists of a 120°
ESA that is backed by an array of seven CEMs for energy-angle
analysis of incident plasma electrons. The energy range
of the instrument is 1 to 1400 eV, the energy resolution
is ~14%, and the FOV is ~12° in azimuth (this varies
somewhat with polar angle) by ~150° in polar angle.
The center of the FOV is centered along a normal to the
spacecraft spin axis and consequently the FOV sweeps out
approximately 94% of the sky during each spin. Data acquisition
of the GEM is synchronized with that of the GIM and also
takes four spins of the spacecraft to execute. A complete
data matrix for the GEM consists of electron counts for
seven polar angles and twenty four azimuthal angles and
twenty energy levels.
Both the GEM and GIM were in continuous operation since shortly after launch in August 2001 until August 4, 2004,
a month before sample re-entry.
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Curator: Aimee Meyer
Updated: November 2009
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